Channel estimation is a key function of most wireless communication systems. It is advantageous to make the channel estimation from Uplink (UL) transmissions also for Downlink (DL) beamforming purposes in some wireless communication systems. The reason is that the network nodes of the radio access network have more antennas than the user devices and by using assumption of channel reciprocity feedback messaging can be avoided. Estimating the channel from UL beacon transmissions therefore allow the channels for multiple receive antennas to be estimated from the same transmissions. Further, with increasing number of antennas at the network node it is possible to schedule more user devices simultaneously, at the cost of increasing use of the spectral resources for channel estimation.
However, the channel estimations are only valid as long as the physical environment remains stationary, or during the coherence time of the radio channel. This depends on the speed with which the user device and scatters in its surrounding are moving. Since the channel estimations consume spectral resources it is preferable to reduce the number of channel estimation transmissions to the minimum required for each user device.
Beamforming in spatial domain Multiple Input Multiple Output (MIMO) techniques requires up-to-date Channel State Information (CSI) to work properly in mobile environment. If CSI gets outdated beamforming transmissions add extra interference to the wireless communication system. Additionally, due to Modulation and Coding Scheme (MCS) selection made with out-dated CSI, the error rate will be increased. The Outer-Loop Link Adaptation (OLLA) for MCS selection does not work due to changes in the beamforming on short time scales. If dynamic scheduling is enabled the interference experienced by a single user device is different at each Transmission Time Interval (TTI).
Network nodes, such as base stations, usually support 2-3 sectors in cellular systems and the sectors are interfering with each other at the border areas. Multi-User Multiple Input Multiple Output (MU-MIMO), Coordinate Multi-Point (CoMP) and other variants of spatial domain MIMO techniques have been specified for 3GPP specifications. These new beamforming MIMO techniques will require more up-to-date CSI compared to older sectored cells. Current mechanisms for obtaining the CSI, such as those in Long Term Evolution (LTE), have not been designed for this purpose.